In reality the picture is more complex, because the prevailing measure of fatness, weight in kilograms divided by height in metres squared, known as the body-mass index (BMI), is imperfect and because bad for you is a vague term.

According to a new study in the Journal of the American Medical Association (JAMA), being a bit too pudgy may in fact reduce the risk of dying in a given period.

Researchers, led by Katherine Flegal of Americas Centres for Disease Control and Prevention, examined nearly 100 studies of more than 2.9m people and 270,000 deaths. Being overweight is defined as having a BMI between 25 and 30. People with a BMI of 30 or more are considered obese.

Dr Flegal and her colleagues observed that obesity was associated with a higher risk of mortality in a given study period. Interestingly, those who were only moderately obese (with a BMI of 30-35) had a 5% lower risk of death than those of normal weight and those who were merely overweight had a 6% lower risk.

The mortality risk was much higher for those with BMI of 35 or above—they had a 29% higher risk of death in a given period than those of normal weight.

Dr Flegal included studies of general populations, not just those in hospital or with specific conditions. But her findings add new fuel to the debate over what is called the obesity paradox. Those with chronic diseases such as diabetes and heart conditions seem to show an inverse relationship between BMI and mortality—that is, being moderately overweight seems to have a protective effect.

Just why this is true remains unclear. It may be because the overweight receive life-prolonging medical care, such as treatment for diabetes and drugs to control heart conditions. It may be that they are better equipped to endure surgery.

Among those who sought angioplasty for coronary artery disease, a higher BMI was linked with a higher rate of survival. Or, as Wolfram Doehner argued in 2010, chronic illness—of any sort, not just that linked to obesity—may be a metabolically demanding state, with the overweight having more energy reserves to meet that demand. Most controversially, Peter Unger and Philipp Scherer have suggested that body fat may in fact be not the cause of the bundle of conditions associated with obesity, like heart and liver disease, stroke and late-onset diabetes, but its consequence.

The body may use fat tissue to mop up excess lipids caused by overeating. These chemicals, which include fats, are needed in small amounts to make cell membranes. But when fat tissue becomes overloaded, the hypothesis goes, those lipids wreak havoc on other parts of the body. More body fat therefore constitutes a useful buffer.

Whatever the explanation, the latest research highlights three important points. First, physicians must think carefully about diet advice for those who already have chronic conditions. Second, the study is yet another reminder that BMI is a poor measure of health. It just about works as a rough gauge of obesity, but does not account for sex, race, age or fitness. And BMI says nothing about the distribution of fat in the body.

Another study in JAMA, published in September, suggested that BMI was not independently linked with diabetes. Fat storage in the lower body even seemed to protect against disease. Visceral fat, packed around the organs, seems to be the most harmful.

Third, the study may bolster the already strong case (see our special report) for governments to prevent even moderate obesity. Relatively plump citizens may indeed pose a particular burden on the state. On the one hand, they run a higher risk than those who are less fat of developing chronic ailments such as heart disease and diabetes that require expensive treatment.

On the other, corpulence may extend life, meaning such treatment may be needed for many extra years. Expanding waistlines could be making people live longer, but sicker.